This invention relates to a process for the production of phthalic anhydride by catalytic oxidation of a mixture of o-xylene or naphthalene and air, separation of the crude phthalic anhydride from the reaction gas by cooling, and purification of the crude phthalic anhydride by thermal pretreatment and rectification under vacuum, wherein the vacuum is maintained by jet pumps.
In the distillative purification of crude phthalic anhydride, it is conventional to employ rectification under pressures of, for example, 20-300 mm. Hg, preferably 40-100 mm. Hg. Heretofore, the vacuum was generally produced by two-stage steam ejector installations or water ring pumps. In the vacuum line leading from the column to the vacuum pump, a sublimator or a sublimate trap is generally disposed, wherein the vapors taken in by the vacuum line are cooled to 50.degree.-70.degree. C. and partially condensed. Despite this precaution, volatile distillation products pass through the vacuum line into the steam ejector or water ring pump where they are dissolved by the cooling water of the steam ejectors and/or by the operating water of the water ring pump. The thus-contaminated water must be purged periodically or continuously, resulting in a wastewater entailing considerable treatment expense. Moreover, the sublimator must be cleaned from time to time to ensure troublefree operation, i.e., the deposits formed therein must be dissolved with a solvent or melted off; furthermore, due to fumaric acid formation in the deposit, difficulties are frequently encountered in this step. Finally, the steam ejectors must be heated to avoid precipitation of the evacuated, organic vapors in the ejector which would otherwise foul the ensuing pumping operation.